Identification of BaO. A12O3.5TiO2 in the Ternary System BaO. A12O3‐TiO2

Solomah, A. G.; Odoj, R.; Freiburg, Christoph

doi:10.1111/j.1151-2916.1983.tb10546.x

“…Later, Guha et al [12] reported the existence of BaO · Al 2 O 3 · 5TiO 2 which is also hexagonal. Solomah et al [13] confirmed the structural data for BaO · Al 2 O 3 · 5TiO 2 . The existence of a mineral with hollandite-type structure, having the formula BaO · Al 2 O 3 · 4 · 5TiO 2 has also been reported [14].…”

Section: Introductionmentioning

confidence: 64%

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd2O3 ⋅ 4TiO2—BaO ⋅ Al2O3 ⋅ 4TiO2

Azough

¹

,

Lowe

²

,

Freer

³

2005

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BaO · Nd 2 O 3 · 4TiO 2 -based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200-1450 • C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al 2 O 3 to the starting mixtures reduced the sintering temperatures by at least 100 • C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε r ) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ f ) towards zero. The addition of 0.5 wt% Al 2 O 3 with 8 wt% Bi 2 O 3 improved densification, increased both ε r (to 88) and Q × f (to 8000 GHz) and moved τ f closer to zero. Ceramics in the system (1 −x)BaO · Nd 2 O 3 · 4TiO 2 + xBaO · Al 2 O 3 · 4TiO 2 exhibited very limited solid solubility. The end member BaO · Al 2 O 3 · 4TiO 2 was tetragonal in structure with the following dielectric properties: ε r = 35; Q × f = 5000 GHz; τ f = −15ppm/ • C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ f were achieved at compositions of approximately 0.14BaO · Nd 2 O 3 · 4TiO 2 + 0.86BaO · Al 2 O 3 · 4TiO 2 , where Q × f = 8200 GHz and ε r = 71.

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“…Later Guha & Kolar (1976) and Guha (1980) presented additional xrd information characterizing a new tetragonal phase 'BaAl2Ti50 14 although the two sets of data clearly had much in common (compare tables 9 and 10). Solomah et al (1983) subsequently obtained superior xrd data (table 11) and refined tetragonal lattice parameters for their own synthetic 'BaAl2Ti50 14\ Ringwood and co-workers have always described the aluminous barium endmember Synroc hollandite as BaAl2Ti60 16, whereas Vance & Agrawal (1982), and have proposed BaAl2Ti50 14 as an alternative. It would appear, however, that both these stoichiometries represent somewhat misleading oversimplifications.…”

Section: Discussion {A)mentioning

confidence: 99%

“…These calculated multiplicities are reasonably consistent with the average multi plicity of 4.98 for aluminous barium hollandites, as determined by from electron diffraction data. The d spacings listed by Solomah et al (1983), which are reproduced in table 11, are the best available for the hollandite Bax 14A12 29Ti5 710 16, and are recommended for reference purposes. These data yield a tetragonal supercell, with a = 9.977 A and c = 14.742 A (= 2.92 A x 5.05).…”

Section: Discussion {A)mentioning

confidence: 99%

“…5Ti02 and accord ingly a stoichiometry of Ba114Al2 29Ti5710 16. Independently, some closely related compounds have been variously interpreted as new phases, for example ' BaAl2Ti40 12 ' (Guha & Kolar 1972) and 'BaAl2Ti50 14 ' (Guha et al 1976;Guha 1980;Solomah et al 1983) despite X-ray diffraction data with strong similarities to that of hollandite.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

[Ba _x Cs _y ][(Ti, Al) ³⁺ _{2
x
+
y} Ti ⁴⁺ _{8-2
x
-
y} ]O ₁₆

Kesson

¹

,

White

²

1986

Proc. R. Soc. Lond. A

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A series of [Ba x Cs y ][(Ti, Al) 3+ 2 x + y Ti 4+ 8-2 x - y ]O 16 hollandites, synthesized at 1250°C and coexisting with ‘reduced ’ rutile, demonstrates complete solid solution between barium and caesium endmembers, and simultaneously between Ti 3+ and Al 3+ . The presence or absence of rutile has only a minor effect on stoichiometry. For barium endmember hollandites ( y = 0) the stoichiometry (i. e. tunnel site occupancy) ranges from 1.08 ≼ x ≼ 1.14, whilst for caesium endmember hollandites ( x = 0) 1.32 ≼ y ≼ 1.51. Neither x nor y correlates with the nature and proportions of trivalent species. An appropriate stoichiometry for the aluminous barium end-member is confirmed as Ba 1.14 Al 2.29 Ti 5.71 O 16 . The composition BaO. Al 2 O 3 . 5TiO 2 yields this same hoilandite, and not the supposed phase ‘BaAl 2 Ti 5 O 14 ’. The phase ‘BaAl 2 Ti 4 O 12 ’ does not exist, while the composition BaO. Al 2 O 3 . 4TiO 2 crystallizes to an assemblage containing the hollandite mentioned above. Reinterpretation of published X-ray diffraction data substantiate these conclusions and are consistent with a 5 c supercell for hollandite. Superlattice ordering in [Ba x Cs y ][(Ti, Al) 3+ 2 x + y Ti 4+ 8-2 x - y ]O 16 hollandites may be commensurate or incommensurate, with typical multiplicity values ( m ) and tunnel-site occupancies ( x + y ) correlating with increasing caesium content per formula unit throughout the series. Barium end-members and barium-rich hollandites, with Cs + ≼ 0.33 and tunnel-site occupancies of 1.03‒1.15 display 4.5 ≼ m ≼ 5.0. Intermediate hollandites with 0.40 ≼ Cs + ≼ 0.70 and tunnel-site occupancies ranging from 1.14 to 1.23 possess superstructures with 5.5 ≼ m ≼ 5.7, whereas caesium endmembers and caesium-rich hollandites have tunnel-site occupancies between 1.12 and 1.51 and 5.9 ≼ m ≼ 6.3. For barium or caesium endmembers, multiplicities fail to correlate with tunnel-site occupancies, but do increase with increasing percentages of molar Al 3+ /(Al 3+ + Ti 3+ ) in the structure. Superlattice periodicity is considerably more sensitive to changes in the barium‒caesium content of tunnel sites than to variation in the nature of the trivalent species. Long-range superlattice order is determined not so much by the tunnel cations as by the trivalent species. With more than about one Al 3+ per formula unit, one-dimensional (uncorrelated) ordering is suppressed, and three-dimensional order occurs almost exclusively. Hollandite superstructures, and thus their stoichiometries, are determined both by mutual repulsion between large cations within individual tunnels, and intertunnel interaction between large cations. The ceramic high-level nuclear wasteform, Synroc, contains a titanate hollandite belonging to the above series. It has been suggested that the capacity of Synroc to immobilize caesium may be impaired if caesium and barium are not incorporated solely in hollandite, but are partitioned between hollandite and additional titanate phases or hollandite-related structures. No such phase has been encountered in the synthesis of the above hollandite series or in Synroc, prepared according to current specifications, because the trivalent species are present in sufficient abundance to allow the incorporation of all barium and caesium in hollandite. Consequently two-component titanates (for example Cs 2 Ti 6 O 13 or Ba 2 Ti 9 O 20 ), do not appear in the phase assemblage. Moreover, the trivalent species do not comprise Al 3+ alone but also include some Ti 3+ , which promotes more favourable structural modifications and kinetics. Furthermore, the phase assemblage includes ‘reduced’ rutile, which effectively prohibits crystallization of two-component titanates with [Ba, Cs]/[Ti] ratios higher than that in hollandite, and also three-component [Ba, Cs] [Ti, Al] 3+ -titanates other than hollandite. When these three criteria are satisfied, the appearance of additional, potentially undesirable phases in the Synroc mineralogy is suppressed, and all barium, caesium (and rubidium) may be successfully immobilized in hollandite.

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